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Nathan Willcox,
Environment Iowa

New Report: Extreme Downpours Up 35 Percent in Iowa

Scientists Link Trend to Global Warming
For Immediate Release

Des Moines, IA— Eleven months after a heavy rainstorm led to record flooding that devastated Council Bluffs, a new Environment Iowa report confirms that extreme rainstorms are happening 35 percent more frequently in Iowa since 1948.

“As the old saying goes, when it rains, it pours—especially in recent years as bigger storms have hit Iowa more often,” said Tyler Pidgeon, Field Organizer for Environment Iowa. “We need to heed scientists’ warnings that this dangerous trend is linked to global warming, and do everything we can to cut carbon pollution today.”

Based on an analysis of state data from the National Climatic Data Center, the new report found that heavy downpours that used to happen once every 12 months on average in Iowa now happen every 9 months on average. Moreover, the biggest storms are getting bigger. The largest annual storms in Iowa now produce 10 percent more precipitation, on average than they did 65 years ago.

Scientists have concluded that the rise in the frequency and severity of heavy rainstorms and snowstorms is linked to global warming. Warming increases evaporation and enables the atmosphere to hold more water, providing more fuel for extreme rainstorms and heavy snowstorms.

Pidgeon pointed to the rainstorm that hit Council Bluffs in August 2011 as an illustration of what more extreme rainstorms could mean for the state. That rainstorm, which dumped 4-6 inches of rain overnight, overwhelmed drainage systems, caused house foundations to collapse, trapped cars and residents in 4-5 feet of rain water in some areas, and led to high water rescues, including a school bus full of children that had become stranded.

The new Environment Iowa report, When It Rains, It Pours: Global Warming and the Increase in Extreme Precipitation from 1948 to 2011, examines trends in the frequency of and the total amount of precipitation produced by extreme rain and snow storms across the contiguous United States from 1948 to 2011. Using data from 3,700 weather stations and a methodology originally developed by scientists at the National Climatic Data Center and the Illinois State Water Survey, the report identifies storms with the greatest 24-hour precipitation totals at each weather station, and analyzes when those storms occurred. The report also examines trends in the amount of precipitation produced by the largest annual storm at each weather station.

Nationally, the report found that storms with extreme precipitation increased in frequency by 30 percent across the contiguous United States from 1948 to 2011. Moreover, the largest annual storms produced 10 percent more precipitation, on average. At the state level, 43 states show a significant trend toward more frequent storms with extreme precipitation, while only one state (Oregon) shows a significant decline.

Key findings for Iowa and the West North Central census region include:

  • Extreme rainstorms and snowstorms are becoming more frequent. Iowa experienced a 35 percent increase in the frequency of extreme rainstorms from 1948 to 2011. In other words, heavy downpours or snowstorms that happened once every 12 months on average in 1948 now happen every 9 months, on average.
     
  • Storms with extreme precipitation increased in frequency by 36 percent in the West North Central census region (IA, KS, MN, MO, ND, NE, SD) during the period studied.
     
  • The biggest rainstorms and snowstorms are getting bigger. The amount of precipitation released by the largest annual storms in Iowa increased by 10 percent from 1948 to 2011.

Pidgeon was careful to note that an increase in the frequency and severity of extreme rainstorms does not mean more water will be available for human use. Hotter temperatures fuel extreme rainstorms by increasing rates of evaporation. At the same time, however, that evaporation increases soil dryness. Moreover, scientists expect that, as global warming intensifies, longer periods with relatively little precipitation will tend to mark the periods between heavy rainstorms. As a result, droughts are likely to become more frequent and severe in some regions of the United States. Currently, more than half of the lower United States is suffering through prolonged drought, aggravated by the fact that the last six months have been the hottest January-June period on record.

According to the most recent science, the United States must reduce its total global warming emissions by at least 35 percent below 2005 levels by 2020 and by at least 85 percent by 2050 in order to prevent the most devastating consequences of global warming. Environment Iowa highlighted two proposals from the Obama administration—carbon pollution and fuel efficiency standards for cars and light trucks through model year 2025, and the first ever carbon pollution standards for new power plants—as critical steps toward meeting these pollution reduction targets.

“How serious this problem gets is largely within our control – but only if we act boldly to reduce the pollution that fuels global warming,” said Pidgeon. “We applaud the Obama administration for their proposals to cut carbon pollution from vehicles and new power plants, and urge them to move forward with finalizing these critical initiatives this year.”